When choosing a UK casino with many high RTP slots, start by checking the game...

https://tiny-wiki.win/index.php/How_Do_I_Use_%27Popularity%27_Filters_Without_Falling_for_Hype%3F

When choosing a UK casino with many high RTP slots, start by checking the game library size and the software providers featured. A larger selection from reputable developers usually means more variety in RTP rates